The field of the invention is the oxo process and the present invention is concerned with an improvement in the yield of the hydroformylation of propene.
The state of the art of the oxo process and the most pertinent improvements thereof may be ascertained by reference to the Kirk-Othmer "Encyclopedia of Chemical Technology" 2nd Edition, Vol. 14 (1967), pp. 373-390, particularly page 379 where it is indicated that propylene (propene) is used to produce butyraldehyde, pages 283-386 and FIG. 4 where hydroformylation and the steps of the oxo process are diagrammed and explained; German Published Application 2,460,784 and U.S. Pat. Nos. 3,501,537 and 4,048,233.
Kirk-Othmer, ibid, Vol. 8 (1966) discloses on page 794 that acid value or acid number is the number of milligrams of potassium hydroxide required to neutralize the free acids in one gram of a substrate.
The acid number of 1 is equivalent to 1.32 mg butyl alcohol.
In the large scale industrial hydroformylation of propene, especially in the presence of cobalt-catalysts, after separating off the n-butyraldehyde and iso-butyraldehyde and the by-products n-butanol and iso-butanol and n-butyl formate and iso-butyl formate by distillation, distillation residues are obtained which in general were hitherto burned. The residues contain, inter alia, n-butyric acid, iso-butyric acid, n-butyric-iso-butyl ester, iso-butyric n-butyl ester, n-butyric n-butyl ester, iso-butyric isobutyl ester, 2-ethylhexenal, 2-ethylhexanal, 2-ethylhexanol, n-butyraldehyde-di-n-butyl acetal and isomeric compounds and the monobutyrates and dibutyrates of 2-ethylhexane-1,3-diol and of 2-ethyl-4-methylpentane-1,3-diol.
The mixture consists of the isomeric butyric acids to the extent of up to 25%, of the butyl esters of butyric acid and butyrates of the dihydroxy compounds to the extent of up to 38% and of the isomeric butyraldehyde dibutyl acetals to the extent of about 10%.
Since in the case of hydroformylation processes carried out on a large industrial scale in general up to 5% of the crude hydroformylation product is obtained, during the distillation, as residues which cannot be utilized directly, there have been no lack of attempts to either refine them or to use them in an economically more favorable manner as a starting material for the production of the synthesis gas, olefin and hydrogen required in the hydroformylation process. According to U.S. Pat. No. 4,048,233, in order to manufacture a synthesis gas, the by-products and waste products of hydroformylation reactions are reacted at temperatures from 600.degree. to 900.degree. C. in the presence of steam and carbon dioxide on nickel-containing catalysts.
According to German Published Application 2,460,784, the esters obtained as by-products are saponified with sodium hydroxide solution or potassium hydroxide solution. The resulting alcohols are distilled off and the free carboxylic acid are obtained from the salts of the carboxylic acids by adding strong aqueous mineral acids. Since equivalent amounts of aqueous alkali and mineral acid are employed in this process, the raw material consumption of aqueous alkali and mineral acid is very high. The aqueous phase obtained contains organic compounds, in addition to large amounts of inorganic salts. This gives rise to high costs in the purification of the effluent.
It was known from U.S. Pat. No. 3,501,537 to hydrogenate the distillation residues from the hydroformylation with the aid of two catalyst beds arranged in series. The corrosiveness of these acid containing mixtures under the hydrogenation conditions, that is to say 230.degree. to 255.degree. C. and 70 to 700 bars, makes it necessary for the hydrogenation apparatus to be of expensive construction. Furthermore, under the energetic hydrogenation conditions claimed, after the addition reaction with hydrogen, acetals are more readily split into a mixture of butanol and butyl ehter than into butanol alone. In addition, the distillation carried out prior to the hydrogenation prevents utilization of high-boiling butyrates of dihydroxy compounds, which remain in the sump of the column and economic use thereof is therefore lost. Moreover, the life of the contact catalyst during the hydrogenation of non-pretreated acid/ester mixtures on only one catalyst is in general very low. Special measures and a significant technical effort are therefore necessary to increase the life of the catalyst.